[ltt-dev] UST clock rdtsc vs clock_gettime
david.goulet at polymtl.ca
Tue Jul 6 14:12:11 EDT 2010
After some talks with Nils from Ericsson, there was some questions about
using the TSC counter and not clock_gettime in include/ust/clock.h
I ran some test after the meeting and was quite surprised by the
overhead of clock_gettime.
On an average run ...
WITH clock_gettime : ~ 266ns per events
WITH rdtsc instruction : ~ 235ns per events
And it is systematic... I'm getting stable result with rdtsc with
standard deviation of ~2ns.
As little as I know on TSC, one thing for sure, with SMP, it becomes
much more "fragile" to rely on it because we don't have assurance of
coherent counters between CPUs and also the CPU scaling policy (ondemand
is default on Ubuntu now). New CPUs support constant_tsc and nonstop_tsc
flags but still a small range of them.
Right now, UST is forcing the use of clock_gettime even if i386 or
x86_64 is used.
Should a change be consider ?
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